Tetradecanoyl phorbol acetate (TPA) alters membrane transport properties characteristic of stimulation of cell proliferation. Among the earlier transport changes induced by TPA (10 to the minus 8th power to 10 to the minus 6th power M) is a 2 to 3 fold increase in 32Pi uptake within 5 to 10 minutes of incubation with 3T3 mouse embryonic fibroblast confluent monolayers. Similarly TPA at these concentrations stimulates 86Rb ion influx. Both influxes are ouabain-sensitive in accord with the idea that an early effect of TPA is to influence Na ion and K ion movements. Since prostaglandins E1 and F2 gamma at similarly low concentrations cause a marked stimulation of 86Rb ion and 32Pi uptake, it is proposed to examine whether phorbol ester and prostaglandin derivatives act at common membrane sites. In view of the decreased uptake of 86Rb ion and 32Pi when 3T3 mouse embryonic fibroblasts become quiescent, and the possibility that 32Pi uptake may be secondary to K ion uptake or (Na ion plus K ion) ATPase, it is of interest to learn whether agents such as tumor promoting phorbol esters which have been reported to promote cell proliferation initiate DNA synthesis by alternating the supply of nutrients such as monovalent cations.